The long-term objective of this work is to understand the mechanisms involved in the de novo formation of blood vessels. One component process, vascular fusion, involves the coalescence of small caliber vessels to form larger caliber vessels. We hypothesize that interactions between vascular endothelial growth factor (VEGF) and integrins AyB3, ayB5, and a5B1 regulate endothelial cell behavior during vascular fusion. Thus we will use time lapse digital imaging to: 1) examine vascular fusion and how differential regulation of VEGF receptors -land -2 influences endothelial cell motility/protrusive activity, and 2) determine which aspects are regulated by VEGF- and/or integrin-dependent mechanisms. An avian embryo culture method will allow microinjection of reagents such as marker antibodies and perturbing reagents (VEGF, VEGF receptor-selective ligands, and functional blocking antibodies). Vessel formation will be monitored at the light optical level of resolution utilizing time-lapse digital analysis and a fluorochromeconjugated endothelial cell marker (QH1). This system will allow for quantitative analysis of processes involved in de novo vessel formation, which is crucial for normal development and is implicated in adult neovascularization under certain pathological conditions.